This invention relates to an improved method and apparatus for optical determination of the saturation temperature of a solution which dissolves a solute.
The saturation temperature meter is an instrument which is necessary when, in organic and inorganic chemical industries and food industry, the saturation temperatures of varying substances are to be determined for the purpose of scientific management of crystals during the eduction of crystals in solutions. No measure established for providing effective determination of saturation temperature has been known to date.
Recently, an improved saturation temperature meter designed for optical determination of saturation temperature has been reported in the International Sugar Journal, Vol. LXXX, 1978, pp 40-43 (published at 23a Easton Street, High Wycombe Bucks, England).
As illustrated in FIG. 4, this temperature meter is composed mainly of a light source 101, a heating unit 100 provided with a light path 104 and a mount 103 for a test cell 102, and a light-receiving unit 106 provided with a light-receiving element 105. The temperature meter is prepared for operation by placing a solution subjected to test in the test cell 102, causing fine crystals of the solute dissolved in the solution to be added to and suspended in the solution to obtain a test specimen, mounting the test cell 102 containing the test specimen on the cell mount 103 and placing the heating unit 100 on top of the light-receiving unit 106. At this point, an empty space 108 occurs between a heat retaining glass 107 disposed in the lower portion of the light-receiving unit 106 and the test cell 102. With the meter so prepared, admission of a light 109 via the light path 104 upwardly from the lower end of the test cell 102 is started and a heater 110 is switched on to effect gradual indirect heating of the test specimen in the test cell 102. As the heating is continued, the temperature of the test specimen increases and eventually reaches a point at which the fine crystals in the test specimen are dissolved. At this point, a change occurs in the light penetrating through the test cell 102 (the amount of light allowed to penetrate through the cell increases because the scattering of light is decreased on dissolution of fine crystals) and, consequently, a heavy change occurs in the amount of light being continuously received by the light-receiving (photoelectric) element 105. This change manifests itself in a change in the amount of electricity being generated in the light-electricity conversion in the light-receiving element 105. In the meantime, the temperature of the test specimen is continuously measured by a temperature measuring unit 111 which is held in contact with the lower surface of the test cell 102. This temperature meter, therefore, tells the saturation temperature of the solution under test by the combination of the point of the aforementioned change in the amount of electricity and the temperature of the test specimen existing at that point.
In the conventional saturated temperature meter such as described above, during the elevation of the temperature of the test specimen, the test specimen in the test cell 102 and the gas in the empty space 108 gain in volume because of the phenomenon of thermal expansion. Consequently, an increased portion of the gas and a small volume of steam issuing from the surface of the test specimen leak into and fill out the empty space 108, giving rise to a state of steam saturation. In this case, the relation between the temperature of the test cell 102 (T.sub.1) and that of the heat retaining glass 107 (T.sub.2) is T.sub.1 &gt;T.sub.2 under normal working conditions. Consequently, part of the steam filling up the empty space 108 comes into contact with the surface of the heat retaining glass 107 and forms dew-condensation thereon. The present inventors' experience tells that where the temperature (T.sub.2) is in the range of from 5.degree. to 10.degree. C., the dew-condensation occurs when the temperature difference (T.sub.1 -T.sub.2) is about 0.2.degree. C., and that even where the temperature (T.sub.2) falls within the range of from 25.degree. to 30.degree. C., the dew-condensation formation ensues when the temperature difference (T.sub.1 -T.sub.2) is about 1.degree. C. When the dew-condensation forms as described above, it causes scattering of the light penetrating through the test cell 102 to impair the accuracy of the determination. If the heating speed is to be lowered enough to preclude the dew-condensation formation, the speed must be brought down to an extreme lever such that the determination requires an excessively long time, the change in the volume of the penetrating light occurs very slowly and the saturation point appears very vaguely.
Further according to the conventional saturation temperature meter, the test specimen is prepared by suspending added fine crystals of the solute in the solution under test. In case where the solution under test has a high purity or where the solution involves a high degree of supersaturation, therefore, the initial crystallization (occurrence of pseudocrystals) either during or after the preparation of the test specimen proceeds very quickly so that the procedure of determination demands high skill or the reproducibility of the determined values is impaired.